The present invention pertains generally to digital telecommunications systems and more particularly to failure detection of digital attenuation apparatus for use in the network of such digital telecommunications systems.
In telecommunications systems, it is necessary to attenuate the signals transmitted between these systems or between a system and subscriber equipment. This operation consists of modifying the signal to be transmitted from one of the above mentioned sources to another. The modification of the signal may include attenuation or amplification. These signals to be transmitted are passed through various impedance modification devices to obtain the desired attenuation or amplification. Historically, these inpedance modification devices were implemented using discrete R-C networks.
With the advent of electronic digital switching, signal padding was accomplished with digital pads. These digital pads usually take the form of read only memory devices which are accessed via the magnitude value of the signal, used as an index to the read only memory. Such digital padding schemes are typified by U.S. Pat. No. 4,220,823, issued to B. G. Littlefield on Sept. 2, 1980.
The read only memory devices are subject to failures of input addressing leads and the output data leads. A fault in either of these two areas may produce a signal having the wrong attenuation factor.
Accordingly, it is the object of the present invention to provide failure detection apparatus for use with digital pads in a digital telecommunications system.